and I was able to get everything connected (except for the USB DVD writer) and worked great, except this one it will not function without an AC adapter. So I have to start the car in order to use anything. Trying to avoid that if possible.

At the moment, I can give up the Gyration keyboard as I don't really use it that often, but for the remaining 3 devices, would it be do-able with a self-powered hub? Or it will run out of juice since I'll have a DVD writer and Powermate?

The TabletPC has 3 USB ports, but the other one is at the top where it doesnt' have enough clearance to plug an USB plug into. The 3rd one is on the opposite side of all the plugs are where I'm trying to avoid - to keep everything nice!

Surely the acessories adaptor is powered when the car is switched to ACC?

Powered hubs are far more reliable and useful than self powered ones.
It has to do with the fact that no single USB device can request more than 500 mA, but every USB device thinks it get 500 mA. Your hub is limited to a total of 500 mA which must be split between the remaining 4 (or whatever) ports plus its own electronics.